1. Field of the Invention
The present invention relates to a frequency shift keying (FSK) signal detector for detecting an FSK signal. More particularly, the present invention relates to an FSK signal detector for detecting an FSK signal through digital processing.
2. Description of the Background Art
Up to now, an FSK signal detector, detecting an FSK signal by digital signal processing, includes a quadrature detection circuit, made up by, for example, an oscillator, a 2/π-phase shifter and a pair of multipliers. The quadrature detection circuit decomposes a received input intermediate frequency (IF) signal by quadrature detection into an I (in-phase) signal and a Q (quadrature) signal, which are baseband signals, and quantizes these I and Q signals, using an analog-to-digital (A/D) converter, to generate digital data representing amplitude information. These data are input to a phase detector having a table indicating the relationship between the amplitudes of the I and Q signals and tan−1θ to generate phase information corresponding to the information of the amplitudes of the I and Q signals. The one-symbol delay difference of the phase information, obtained from the amplitudes of the I and Q signals, is then found out to determine how the phase has shifted, in order to output a detection signal.
For example, if, if a phase θ1 is detected with a symbol S1, a phase θ2 is detected with a symbol S2, next following the symbol S1, and the phase difference θ2-θ1 is positive, then the phase has advanced during the time of transfer from the symbol S1 to the next symbol S2. If the phase θ1 is detected for the symbol S1, the phase θ3 is detected for the next following symbol S3 and the phase difference θ3-θ1 is negative, then the phase has lagged during the time of transfer from the symbol S1 to the next symbol S3, so that the frequency has become lower at symbol S3 than that at symbol S1. In this manner, the state-of-art FSK signal detector frequency-detects the FSK signal by reading the frequency shift between symbols for phase shift between symbols.
For simplifying the configuration of a receiving circuit, adapted for receiving an FSK signal, it has also been proposed to convert an output of a receiving analog circuit into a bi-level signal and to detect a frequency component by digital signal processing by way of performing demodulation.
In Japanese Laid-Open Patent Publication 36924/1997, there is disclosed a multi-level FSK receiving device for taking out the frequency information to high accuracy.
In the state-of-art FSK signal detector, the frequency detection is carried out on the basis of amplitude information of I and Q signals. Thus, if the amplitudes of the I and Q signals are varied by, e.g. noise, such variations in the amplitudes affect the characteristics of the frequency detection. The state-of-art FSK signal detector also suffers from the problem that it is necessary to use components, such as A/D converters, and that, since the amplitude information of the I and Q signals are represented with plural bits, the circuit is complicated in constitution.
In an FSK signal detector, disclosed in U.S. Patent Application Publication US 2005/0105653 A1 to Mizuno, it may be contemplated that, if a signal other than frequency components provided in a correlator is received, not only the frequency components contained in such signal but also the phase components of an input signal are simultaneously output, such that, even in the case there are no variations in frequency components, the output of the FSK signal detector may undergo fluctuations.